In co-pending application of Lee R. Zehner et al, Ser. No. 564,242, filed Apr. 2, 1975, entitled PROCESS FOR THE PREPARATION OF OXALATE ESTERS and incorporated herein by reference, there is disclosed a process for the oxidative carbonylation of alcohols at elevated temperatures and pressures in the presence of palladium (II), rhodium (III), platinum (II) and copper (I) or (II) catalysts and a stoichiometric amount of a copper (II) or iron (III) oxalate, sulfate, acetate or trifluroacetate oxidant salts and an aliphatic, aromatic or heterocyclic amine or ammonia to produce a predominant amount of the desired oxalate ester. Various counterions and ligands of the metal catalysts may also be employed.
The present invention is directed to an improved process for the oxidative carbonylation of alcohols, as disclosed in the above described co-pending application, wherein an amine salt is added to the reaction mixture or formed in situ in the reaction mixture by the addition of an acid as sulfuric acid or acetic acid, so that the reduced form of the stoichiometric copper (II) oxidizing salt resulting from the oxidative carbonylation can be reoxidized with oxygen or oxygen-containing gas and under mild conditions. By this process the synthesis of oxalate esters can be carried out as a cyclic process providing high yield of the oxalate esters and exceeding the yield of carbonates.
The addition of excess amine salts to the reduced form of the copper (II) oxidizing salt resulting from the oxidative carbonylation allows the reoxidation (oxygen-oxidation) to take place without precipitation of basic copper salts. The basic copper salts were shown to be insoluble in the reaction solvents and therefore to be ineffective oxidants for the oxidative carbonylation. In the presence of the additional amine salt, the reoxidation coproduct, water, can be removed, for example by chemical reaction or by azeotropic distillation. Without the excess amine salt, the overall synthesis of oxalate esters cannot be accomplished in a cyclic manner.
U.S. Pat. No. 3,114,762 discloses a method for the preparation of alkyl carbonates by reacting carbon monoxide with an alcohol in the presence of platinum or palladium chloride and in the added presence of an oxidizing salt for reoxidizing the catalyst in situ. The reaction is carried out at temperatures of from 20.degree. C. to 100.degree. C. and carbon monoxide pressures of 1 to 500 atmospheres. Runs carried out for comparison even at higher temperatures only resulted in trace amounts of the oxalate.
U.S. Pat. No. 3,393,136 describes a process for the preparation of oxalates by contacting carbon monoxide at superatmospheric pressure, with a saturated monohydric alcohol solution of a platinum group metal salt and a soluble ferric or cupric salt (redox agent) while maintaining the salts in a highly oxidized state by the simultaneous introduction of oxygen or the application of a direct current electrical potential to the reaction zone. When oxygen is employed, explosive mixtures of oxygen and combustible organic vapors in the gas phase must be avoided and water scavengers or dehydrating agents such as alkyl orthoformic acid esters must be added to the liquid phase to prevent the accumulation of water.
In a recent article by Donald M. Fenton and Paul J. Steinwand, Journal of Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704, a general mechanism for the oxidative carbonylation of alcohols to yield dialkyl oxalates using a palladium redox system, oxygen and dehydrating agents has been proposed. In the absence of the necessary dehydrating agent, a large amount of carbon dioxide is formed and oxalates are not produced. The necessity of the iron or copper redox system during the oxalate synthesis is emphasized.
A recent West German Pat. No. 2,213,435 discloses a method for the synthesis of oxalic acid and oxalate esters in water and alcohol respectively. A platinum group metal salt, a salt of a metal more electropositive than the platinum group metal, e.g. copper (II) chloride and an alkali metal salt comprise the catalyst. Oxygen in stoichiometric amounts was employed as the oxidant. A disadvantage of such reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect reaction. Under non-explosive conditions only trace amounts of oxalate can be obtained.
Many important commercial applications have been developed for the oxalate products of this invention, for example, as cellulose ether or ester and resin solvents, as dye intermediates and the preparation of pharmaceuticals.
The process of the present invention provides a means of carrying out the oxidative carbonylation of alcohols as a cyclic process with the reoxidation of reduced copper (II) oxidant salts and thus obtain high yield selectivity to the oxalate esters. Carbonates esters and carbon dioxide associated with such reactions are minimized by a critical regulation of the amine catalyst, and oxidant anions and by maintaining the reaction mixture substantially anhydrous.
Other advantages of the present invention, as compared to known prior art processes are (1) elimination of hazardous operational conditions by avoiding explosive mixtures of oxygen and carbon monoxide, (2) avoiding any necessity for using dehydrating agents as no water is formed as a result of the instant oxidative carbonylation process; when air or O.sub.2 is used as the oxidant water and CO.sub.2 are formed and the presence of water always decreases the yield of oxalate and increases the CO.sub.2, (3) avoiding the use of large amounts of corrosive chloride ions, (4) ease of recovery and reoxidation of the metal salts in a stream of air or oxygen for reuse in the oxidative carbonylation process and (5) the ability to employ in the process as catalysts the more readily available copper salts in place of the more expensive platinum group metal salts.